<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        :root{
            --main-color: red;
            background-color: lightblue;
            color: white;
        }

        html{
            padding: 0px;
            margin: 0px;
            height: 100%;
            background-color: lightgreen;
            color: red;
        }

        header{
            color: var(--main-color)
        }

    </style>
</head>
<body>
    <header>HEADER</header>
    <footer> FOOTER </footer>
</body>
</html>